Candida albicans is an important human pathogen This fungus is a member of the normal body flora but under certain conditions, for instance when there is a defect in immunity, C. albicans is no longer benign. Instead it becomes a dangerous pathogen that colonizes different tissues, causing grave illness and death. C. albicans is a major concern in hospitals where it is increasingly being identified as the causal agent in nosocomial infections. As yet, there are few effective drugs against C. albicans. 
C. albicans has a diploid genome and no known sexual cycle. Consequently, although C. albicans is of great scientific interest, classical genetic studies have been very challenging. Furthermore, modern genetic reporter systems have been hampered by the difficulties in heterologous protein expression in C. albicans. Expression of yeast-enhanced green fluorescent protein in C. albicans has been reported (Cormack et al., Yeast-enhanced green fluorescent protein (yEGFP): a reporter of gene expression in Candida albicans. Microbiology 143: 303-311 (1997)), but additional fluorescent markers that are well expressed in yeast, particularly in C. albicans, and useful for analysis of expression, protein localization, protein-protein interaction, and the like are desired.
Red fluorescent protein (RFP) was first isolated and sequenced from a Discosoma sp. (see, e.g., Matz et al., Nature Biotech. 17:969 973 (1999), Gross et al, Proc. Nat'l. Acad. Sci. USA 97:11990 11995 (2000)). The crystal structure of red fluorescent protein shows it to be a tetrameric protein (Wall et al., Nat. Struc. Biol. 7:1089 (2000); Yarbrough et al., Proc. Nat'l Acad. Sci. USA 16:462-467 (2000)). A humanized variant RFP has also been engineered (Clontech, “DsRED™”).
RFP, like other fluorescent proteins, is useful as a reporter molecule for a variety of bioassays. Directed mutagenesis experiments that led to development of the bright monomeric mRFP variants used a DsRFP template (DsRed-N1) that had been codon-optimized for expression in human cells (Bevis, et al., Rapidly maturing variants of the Discosoma red fluorescent protein (DsRed). Nature Biotech. 20: 83-87 (2002); Campbell et al., A monomeric red fluorescent protein. Proc. Nat'l. Acad. Sci. USA 99: 7877-7882 (2002)). The monomeric variants of the tetrameric Discosoma sp. RFP are widely used as fluorescent protein tags in mammalian cells, but are not well expressed in S. cerevisiae and other yeast and fungi with. AT-rich genomes, and expression in C. albicans of RFP has not been possible thus far.